Sub-surface target detection systems may be based on frequency domain measurements. One significant advantage of such Frequency-Domain Electromagnetic (FEM) systems over Time-Domain Electromagnetic (TEM) systems is that with FEM systems it is possible to achieve a larger dynamic range in the presence of noise because of the narrow-band filtering in such systems. Since the operational bandwidth can be controlled, FEM systems can use much higher transmitter (TX) power without violating FCC regulations when operated at low frequencies below 9 kHz and at higher frequencies within the Industrial, Scientific and Medical (ISM) bands. Another advantage of the frequency domain is that FEM systems measure both in-phase and quadrature-phase components of the data, both of which can contribute to target characterization. TEM methods typically measure only during ‘off-times’ (i.e., when the primary field is turned off). This is equivalent to measuring only the quadrature-phase data and the important in-phase data are lost. FEM measurements can be made at multiple frequencies and over a very broad bandwidth, which also allows for a more detailed target characterization.
A major limitation of the FEM technique has been that measurements are made in the presence of the very strong primary field. Overcoming this problem presents the greatest challenge in achieving high performance in FEM systems. The amount of reduction in the primary field at the location of the receiver (RX) antenna directly contributes to an increase in dynamic range of the FEM system.
One common approach to reducing the primary field at the RX antenna is to orient the TX and RX coils orthogonal to each other (typically referred to as “geometric nulling”). A second common approach is to use a nulling signal from a third coil (typically referred to as “electrical nulling”). The long-term stability of the geometric or electric null is important for system performance. Because the primary field is large, small variations in the geometric alignment of the TX and RX antennas, and any nulling coils can cause a substantial change in the level of signal measured at the RX antenna. Any change in signal level due to geometric misalignment or error in an electrical nulling signal can mask actual target related signals. Therefore, the sensitivity and the measurement accuracy of state-of-the-art FEM systems are largely dependent on the degree of stability of the null over long periods of time, which is commonly characterized by the long-term drift of the system. The presence of the large primary field is especially challenging when trying to map deeply buried resistive bodies, since resistive bodies typically exhibit much smaller secondary fields than conductive bodies.
Techniques which permit the removal (i.e., nulling) of the large primary field as well as the secondary field from a layered earth and greatly reducing the temporal drift that is exhibited in commercially-available FEM systems have been described in Sternberg et al., “A New High-Sensitivity Subsurface Electromagnetic Sensing System Part I—System Design,” Journal of Environmental & Engineering Geophysics 13:247-261 (2008), and Krichenko et al., “A New High-Sensitivity Subsurface Electromagnetic Sensing System Part II—Measurement Results,” Journal of Environmental & Engineering Geophysics 13:263-275 (2008) all of which are incorporated herein by reference. These techniques can provide enhanced capabilities for the mapping of buried targets. The effects of drift can be compensated with an antenna array rotation referred to generally as an Alternating Target Antenna Coupling (ATAC) method. In such systems a TX antenna and an RX antenna are fixed with respect to each other in a null configuration to form a null coupled array. The null coupled array can have a vertical or horizontal orientation with respect to the earth's surface, and a series of relative measurements at different rotational positions of the array can be used to compensate instrument drift and significantly improve long-term measurement stability and accuracy. An electrical rotation is possible, but electrical rotation-based techniques are generally less sensitive and provide a much lower dynamic range than can be achieved with mechanical rotation over short time intervals.
One of the advantages of such ATAC methods is that measurements can be made in any direction about an array rotation axis. Data obtained from multi-directional measurements can be used to determine the location of a target in three-dimensional (3-D) space, as well as to discriminate between multiple targets. The close spacing between the TX and RX antennas make ATAC techniques particularly applicable to shallow targets such as unexploded ordnance (UXO) and utility lines.
Successful mapping of deeper targets requires a large separation between the TX and RX antennas or a large diameter TX antenna. In some applications, TX/RX antenna dimensions of a few kilometers may be required to map targets at corresponding depths. At large separations, a much larger transmitter moment must also be used to obtain an adequate signal-to-noise ratio, and ATAC systems are impractical due to practical difficulties in securing a large TX antenna with a large antenna moment and a widely-spaced RX antenna to a rigid, rotatable axis to form a nulled array. For at least these reasons, new approaches are needed, particularly for the measurement of deep objects.